Take Over and Fix Our Existing Web App - Developer Left, No Documentation (React + Node.js)

We have a web app for our business. Our customers place orders through it and our staff use the admin part every day. React in front, Node.js and PostgreSQL behind. One developer built it over two years. He left in spring and does not answer anymore. Nobody here can touch it now. There is no documentation. The code is in his GitHub and on the server, we have all the passwords. He deployed by hand on the server, we do not know how. Two things are broken since he left: the export of orders to Excel gives an error, and the email notifications to customers stopped. We are afraid to change anything, because we do not know what depends on what. We need someone to take this over: understand the code, fix these two things, set up a normal way to deploy, and write down how it all works, so we are never in this situation again. After that we want to continue with small changes every month. Budget 1500 USD for the takeover part. If you see it does not fit, say what you would do first and what can wait.